1. Field of the Invention
The present invention relates to methods for manufacturing multilayer ceramic substrates, and more particularly, relates to a method for manufacturing a plurality of multilayer ceramic substrates, which comprises a step of forming a multilayer mother substrate and a subsequent step of dividing the multilayer mother substrate so as to obtain the plurality of multilayer ceramic substrates.
2. Description of the Related Art
Multilayer substrates are formed of a plurality of ceramic layers laminated to each other. The multilayer ceramic substrates are provided with various types of wiring conductors. As the wiring conductors, for example, internal conductive films extending along predetermined interfaces between ceramic layers or via hole conductors penetrating through predetermined ceramic layers may be formed inside the multilayer ceramic substrates, and external conductive films extending on the external surfaces of the multilayer ceramic substrates may also be formed.
The multilayer ceramic substrate is used for mounting semiconductor chip elements and other chip elements thereon and is also used for connecting these electronic elements to each other. The wiring conductors are used as electrical paths for forming interconnection among these elements.
In addition, passive elements such as a capacitor element or an inductor element may be formed inside the multilayer ceramic substrate in some cases. These passive elements may be formed of parts of the internal conductive films or the via hole conductors described above.
The multilayer ceramic substrates are used as, for example, LCR hybrid high-frequency devices in the field of mobile communication terminal apparatuses; composite devices formed of active elements, such as a semiconductor IC chip, and passive elements, such as a capacitor, an inductor or a resistor, in the field of computers; or simple semiconductor IC packages.
In particular, the multilayer ceramic substrates have been widely used for forming various electronic devices, such as power amplifier (PA) module substrates, radio-frequency (RF) diode switches, filters, chip antennas, various package devices and hydride devices.
In order to form a multilayer ceramic substrate having enhanced multifunctionality, higher integration density and improved performance, it is effective that the wiring conductors be densely formed.
Since a firing step must be performed in order to obtain the multilayer ceramic substrate, the shrinkage of the ceramic caused by firing occurs in three directions X, Y, and Z. The shrinkage is likely to occur nonuniformly in the entire multilayer ceramic substrate, and the dimensions between the X direction and the Y direction may become different by approximately 0.4% to 0.6%. As a result, warping of the multilayer ceramic substrate occurs. Consequently, the positional accuracy of external conductive films may be degraded, and in addition, undesired deformations, strains or disconnections of internal wiring conductors may also occur in some cases. These defects in the wiring conductors interfere with the trend towards higher wiring density of conductors described above.
Accordingly, a so-called non-shrinkage process has been proposed in which the firing shrinkage in the direction parallel to the main surface can be substantially prevented in when a multilayer ceramic substrate is formed.
In a method for manufacturing a multilayer ceramic substrate by the non-shrinkage process, a low-temperature sinterable ceramic powder which is sinterable at a temperature of, for example, 1,000° C. or less is prepared as an insulating ceramic material, and in addition, an inorganic powder which functions to suppress the shrinkage and which is not sintered at the sintering temperature of the low-temperature sinterable ceramic powder is also prepared. When a green laminate which will be fired to form the desired multilayer ceramic substrate is produced, shrinkage suppression layers containing the non-sinterable ceramic are disposed so as to sandwich a plurality of ceramic green sheets which are laminated to each other and which contain the low-temperature sinterable ceramic material, and on which the ceramic green sheets, wiring conductors are provided.
The green laminate thus formed is then fired. During this firing step, reaction layers approximately 2 to 3 μm thick are formed at the interface portions between the ceramic green layers and the shrinkage suppression layers, and each reaction layer serves to adhere the ceramic green sheet and the shrinkage suppression layer to each other. In addition, the shrinkage does not substantially occur in the shrinkage suppression layer since the inorganic powder contained in the shrinkage suppression layer is not substantially sintered. Accordingly, the shrinkage suppression layers restrain the ceramic green layers from shrinking, and hence, the ceramic green layers substantially shrink only in the Z direction, that is, the thickness direction, but are restrained from shrinking in the X and the Y directions, that is, the direction parallel to the main surface. As a result, the multilayer ceramic substrate described above obtained by firing the green laminate is not susceptible to nonuniform deformation, the warpage thereof is decreased, the defects of the wiring conductors described above are unlikely to occur, and hence, higher wiring density of the conductors can be realized.
The shrinkage suppression layers described above are removed after firing.
When a multilayer ceramic substrate is produced, a so-called multiple formation method is used in order to increase the production efficiency and comprises a step of forming a multilayer mother substrate which includes a plurality of multilayer ceramic substrates and a step of dividing this multilayer mother substrate along predetermined dividing lines so as to obtain the plurality of multilayer ceramic substrates.
In order to efficiently divide a multilayer mother substrate in the multiple formation method, the multilayer mother substrate preferably has cut-in grooves along predetermined dividing lines. In the case in which the cut-in grooves are provided in accordance with a so-called chocolate break configuration, the multilayer mother substrate may be divided along the predetermined dividing lines when simply bent.
In the multiple formation method, the shrinkage in a firing step is also likely to occur nonuniformly in the whole multilayer mother substrate, the dimensions between the X direction and the Y direction may become different from each other, and warping of the multilayer mother substrate may occur in some cases. Accordingly, the non-shrinkage process described above is preferably used in the multiple formation methods.
In Japanese Patent No. 2856045, a green composite laminate 4 provided with cut-in grooves 6 is disclosed, the green composite laminate 4 comprising first and second shrinkage suppression layers 2 and 3 and a green (i.e., an unfired state) multilayer mother substrate 1 provided therebetween, as shown in FIG. 1 hereof.
Wiring conductors provided for the green multilayer mother substrate 1 are not shown in FIG. 19, and the dimension in the thickness direction thereof is exaggeratedly in the figure.
The green multilayer mother substrate 1 comprises a plurality of ceramic green layers 7 containing an insulating ceramic powder such as a low-temperature sinterable ceramic powder, and these ceramic green layers 7 are formed of a plurality of ceramic green sheets laminated to each other.
The shrinkage suppression layers 2 and 3 contain an inorganic powder that is not sintered at the sintering temperature of the insulating ceramic powder of layers 7. The first and the second shrinkage suppression layers 2 and 3 are each formed of, for example, a predetermined number of inorganic material green sheets 8 which contain an inorganic material and which are laminated to each other.
In order to form the green composite laminate 4, the green multilayer mother substrate 1 is first formed. Next, cut-in grooves 6 are formed in at least one of the major surface sides the green multilayer mother substrate 1. When the cut-in grooves 6 are formed, the green multilayer mother substrate 1 is compressed in the lamination direction of the formed cut-in grooves 6 in order to avoid displacement that may occur among the plurality of ceramic green layers 7.
Next, the first and the second shrinkage suppression layers 2 and 3 are formed by laminating the inorganic material green sheets 8 so as to sandwich the green mother substrate 1, and as a result, the green composite laminate 4 is formed. Subsequently, the whole green composite laminate 4 is again compressed in the lamination direction.
Next, the green composite laminate 4 is fired under the conditions in which the insulating ceramic powder contained in the ceramic green layers 7 is sintered and the inorganic powder contained in the shrinkage suppression layers 2 and 3 is not sintered. Accordingly, the sintered multilayer mother substrate 1 provided between the first and the second shrinkage suppression layers 2 and 3 is obtained. Subsequently, the shrinkage suppression layers 2 and 3 are removed, and hence, the sintered multilayer mother substrate 1 is obtained.
Next, the sintered multilayer mother substrate 1 is divided along the cut-in grooves 6, and hence, the plurality of multilayer ceramic substrates is formed.
In addition, a green composite laminate comprising shrinkage suppression layers and a green multilayer mother substrate provided therebetween is disclosed in Japanese Unexamined Patent Application Publication No. 2000-176928. In this publication, a process is described in which cut-in grooves are provided in the green composite laminate at the two major surfaces sides, the shrinkage suppression layers are further formed so as to cover the cut-in grooves, the laminate thus formed is fired, the shrinkage suppression layers are then removed so as to obtain the sintered multilayer mother substrate, and this multilayer mother substrate is then divided along the cut-in grooves.
However, since the step of forming the cut-in grooves 6, which is different from the previously described in the Unexamined Patent Application Publication step, must be performed in the course of laminating steps for forming the green composite laminate 4 according to the method disclosed in U.S. Pat. No. 2,856,045, the laminating steps and the cut-in groove forming step cannot be efficiently performed, and as a result, the productivity of the multilayer ceramic substrate is decreased.
On the other hand, since the cut-in grooves are formed in the green composite laminate at the two major surface sides according to the method disclosed in Japanese Unexamined Patent Application Publication No. 2000-176928, and the sintered multilayer mother substrate still has the cut-in grooves at the two major surfaces sides, the division of the sintered multilayer mother substrate can be smoothly performed for forming the plurality of multilayer ceramic substrates. However, since the shrinkage suppression layers must be further formed so as to over the cut-in grooves provided in the green composite laminate at the two major surface sides, the process becomes complicated.
In addition, a method for forming external terminal electrodes on side surfaces of the multilayer ceramic substrate has not been described at all in the two publications described above.
Concomitant with the trends towards higher wiring density and miniaturization of multilayer ceramic substrates, it has been strongly desired in recent years that the positions and the dimensions, such as the widths, of the external terminal electrodes be formed with high accuracy. The reason for this is that when the accuracy is degraded and a plurality of external terminal electrodes are provided, short-circuiting may occur between adjacent external terminal electrodes, or electrical connections of a mother board to multilayer ceramic substrates mounted thereon may not be properly obtained in some cases. In particular, when a plurality of external terminal electrodes is plated, and the plating films are abnormally deposited, the problem of short-circuiting between the adjacent external terminal electrodes becomes more serious.